The efficiency of the motor-fan unit is an important factor when it comes to minimize losses in a vacuum cleaner. A part of the fan system where present systems show unnecessary losses is in the air guiding system. Usually, a diffuser is present for deceleration of air ejected from an impeller in a controlled manner, in this way transforming the dynamic pressure created by the impeller into static pressure. Diffusers in vacuum cleaners are either arranged axially or radially. The construction of the diffuser is very important as it affects the efficiency of the vacuum cleaner. A highly efficient diffuser can increase the volume of air being moved or reduces the power required to move the same volume of air. Hence, the desire for a more efficient diffuser is obvious.
In EP 1 878 376 a vacuum cleaner with a radially arranged diffuser is provided. In the known arrangement, the efficiency is increased by changing the inlet angles of the vanes in the diffuser, when combined with changing the return guide vane angles. Radially arranged diffusers result in a vacuum cleaner with large diameter and therefore axially arranged diffusers are preferred when a more compact design is desired. For example in handheld vacuum cleaners where size is an important factor, axial diffusers allow for a design with a smaller outer diameter than radially arranged diffusers.
U.S. Pat. No. 6,442,792 describes a vacuum cleaner with a mixed flow impeller directly connected to an electric motor and with an axial diffuser arranged on the downstream side of the impeller.
A general problem in diffusers is that deceleration of air should be as smooth as possible to minimize losses. By increasing the flow area in the diffuser air channels little by little smooth deceleration is achieved. This is easier to achieve if the air channels are relatively long. A problem when producing diffusers with long channels is that the production tools end up to be very complex. For example, when producing diffusers by injection moulding, the injection moulding tool needs to be extremely complex to produce a diffuser provided with air channels long enough to provide smooth deceleration of air. Another problem arising in long flow channels and in flow channels where the cross sectional area is increased, is boundary layer separation; the air flow will separate from the flow surface it follows, resulting in an increased flow resistance and increased losses. In an arrangement with one diffuser row with relatively long vanes there is a risk that the boundary layers are decelerated and stop thereby creating separation.
Further, for battery operated vacuum cleaners where the available energy usually is limited either by cost and/or space restrictions, there is a need for a compact efficient motor-fan unit with as low losses as possible. Accordingly, there is a need for an improved vacuum cleaner providing both a compact design as well as an efficient fan system with low losses.